CCD image sensors and CMOS image sensors have been widely used as solid-state image sensing elements. In CCD image sensors, light is converted to signal charges by photoelectric conversion elements arranged in pixels, and the signal charges are read out from all the pixels simultaneously and transferred to CCD. The transferred signal charges are converted to electrical signals and then outputted. On the other hand, in CMOS image sensors, light is converted to signal charges by photoelectric conversion elements arranged in pixels, the signal charges are amplified and converted to electrical signals by each pixel and then outputted. CMOS image sensors have the unique feature of being capable of reading out a part of the pixel region of an image sensing unit (hereinafter referred to as “partial readout”). CCD image sensors, which read out the image signals of the entire pixel region at a time, do not have this feature.
FIG. 25 is a schematic diagram illustrating an electronic zoom utilizing partial readout which is a feature of CMOS image sensors (hereinafter referred to as “electronic zoom”). Reference numeral 1301 denotes an effective pixel region of a CMOS image sensor in which a×b pixels are arranged. In this case, the electronic zoom magnification is one (×1). Reference numeral 1302 denotes a pixel region that is read out at an electronic zoom magnification of 2 (×2), at which (a/2)×(b/2) pixels are read out. Reference numeral 1303 denotes a pixel region that is read out at an electronic zoom magnification of 3 (×3), at which (a/3)×(b/3) pixels are read out. The image signal outputted from the CMOS image sensor and converted to a digital signal by an A/D converter (not shown) is divided into a plurality of blocks arranged according to the Bayer arrangement, each block of which consists of a repetition of R, G, G and B, as shown in FIG. 26. Color evaluation values Cx, Cy and Y are calculated for each block using Equations (1) below.Cx=(R−B)/Y Cy=(R+B−2G)/Y Y=(R+G+B)/2  Equations (1)
The color evaluation values Cx and Cy for each block calculated using Equations (1) are compared with a preset white detection region.
FIG. 27 is a graph illustrating the white detection region. A white detection region 101 is determined as follows. First, a white object such as a white reference panel (not shown) is captured using light sources having different color temperatures ranging from high to low at a given color temperature interval. The color evaluation values Cx and Cy are then calculated from Equations (1) using the signal values obtained from the image sensing unit. The color evaluation values Cx and Cy obtained from a light source are plotted on the horizontal axis and the vertical axis, respectively (i.e., Cx on the horizontal axis and Cy on the vertical axis). The plotted points are connected by straight lines, or approximated using a plurality of straight lines. Thereby, a white detection axis 102 that extends from a high color temperature to a low color temperature is formed. For the same white, there may be slight spectral differences, so that the white detection axis 102 is allowed to have some width along the direction of the Y axis. This region is defined as a white detection region 101.
If the calculated color evaluation values Cx and Cy fall within the white detection region 101, that block is assumed to be white. For each block having been assumed to be white, the integral values (SumR, SumG and SumB) of the color pixels of the block are calculated. Using the calculated integral values and Equations (2) given below, white balance gains (kWB_R, kWB_G and kWB_B) for the colors R, G and B, respectively, are calculated (see Japanese Patent No. 03513506 and Japanese Patent Laid-Open No. 2003-244723).kWB—R=1.0/SumR kWB—G=1.0/SumG kWB—B=1.0/SumB  Equations (2)
However, the conventional white balance adjustment for CMOS image sensors suffered the following problem. For example, the color evaluation values of a white object under sunlight are distributed as indicated by a region 103 shown in FIG. 27. If a human face is captured in a close-up manner under a light source having a high color temperature (e.g., sunlight) using an electronic zoom utilizing the partial readout when a moving image mode or EVF mode is set, the color evaluation values of the human complexion are distributed as indicated by a region 105. The region 105 almost agrees with a region 104 where the color evaluation values of a white captured under a light source having a low color temperature (e.g., a white tungsten light source) are distributed. For this reason, when the skin color accounts for a large proportion, as in the case where a human face is captured in a close-up manner, the skin color is sometimes judged erroneously to be the one captured under a light source having a color temperature lower than the actual color temperature.